For many signal processing applications, a high-speed sample and hold circuit plays an important role in converting incoming analog signals into quantized forms which are convenient for further processing and transmission. Conventional electronic sample and hold circuits have been demonstrated on both silicon and GaAs substrates at sample rates higher than 1 giga-sample /second, such as discussed by K. Poulton et al., in their article entitled "A 2 GHz HBT Sample and Hold" GaAs IC Symposium, pp. 199-202, 1988, and by K. Poulton et al., in their article entitled "A 1-GHz 6-bit ADC System," IEEE J. Solid-State Circuits, Vol. SC-22, pp. 962-970, 1987. The circuits of these two publications have the potential of array integration and integration with other electronic signal processing circuits. However, they are subject to the problems associated with timing jitter and other difficulties, especially at high sampling rates.
On the other hand, optical circuits have been regarded as advantageous in terms of realizing very high speed and parallelism. Presently, an all-optical sample and hold circuit has not been achieved as efficient three-terminal active optical devices are still being developed. Instead, optoelectronic sample and hold circuits using optical devices for signal switching and electronic devices for signal processing have been widely studied, such as those referred to in the articles by D. H. Auston entitled "Picosecond Optoelectronic Switching and Gating in Silicon," Appl. Phys. Lett., Vol. 26, pp. 101-103, 1975, F. J. Leonberger et al. in the article entitled "High-Speed InP Optoelectronic Switch," Appl. Phys. Lett., Vol. 35, No. 9, pp. 712-714, 1979, and the article by I. Yao et al. entitled "High-Speed Optoelectronic Track-and-Hold Circuits in Hybrid Signal Processors for Wideband Radar," Picosecond Electronics and Optoelectronics, Proceedings of the Topical Meeting, Lake Tahoe, Springer-Verlag Series in Electrophysics 21, pp. 207-211, 1985, and the article by E. A. Chauchard et al. entitled "Repetitive Semiconductor Opening Switch and Application to Short Pulse Generation," Laser and Particle Beams, Vol. 7, Part 3, pp. 615-626, 1989.
A typical example of an optoelectronic (OE) sample and hold circuit is shown with its functional schematic in FIG. 1 as a conventional direct OE sample and hold circuit 10. This general type of circuit was proposed for wideband radar signal processors by I. Yao et al. in their article mentioned above, and was noted for its simplicity. Laser pulses 11 are utilized to activate an OE switch 12 into the on (or closed) state at position 13. However, this circuit has several major drawbacks. First, with the switch in the ON state, the hold capacitor, C.sub.h (15), is charged directly by an input signal, V.sub.in, from source 16 which can be quite small. Second, the time required to turn off (or open) the EO switch 12 at position 14 depends on the charge recombination, transport and photoconductive gain mechanism. Thirdly, it is typically difficult to achieve suitably low R.sub.on and high R.sub.off for adequate precision at high sample rates. Moreover, the operation of this circuit is limited by a reduction in the extinction ratio of laser pulse 11 at high repetition rates. The limitations of this conventional OE circuit, particularly with respect to the circuit of this invention, will be discussed in detail below.
Similar difficulties also have been encountered in the development of an all-electronic sample and hold circuits. A popular solution is to use the concept of a Wheatstone Bridge to differentiate between the on and off states. The optoelectronic approach becomes advantageous when the sample and hold sequence in this electronic switch type (bridge) circuit is controlled by activating OE switches with laser pulses. This OE bridge-type sample and hold circuit provides more charging capabilities than corresponding direct OE circuits. The periodicity of the laser pulses can be accurate to within the subpicosecond range as discussed by A. J. Taylor et al., in their article entitled "Timing Jitter in Mode-Locked and Gain Switched InGaAsP Injection Lasers," Appl. Phys. Lett., Vol. 49, pp. 681-683, 1986. Good timing control of the laser pulses can be achieved using optical fiber delay lines formed by cleaving and wet etching, see the article by D. J. Albares et al., entitled "Optoelectronics Time Division Multiplexing," Electronics Letters, Vol. 23, pp. 327-328, 1987.
Thus, a continuing need exists in the state of the art for a new bridge-type OE sample and hold circuit based upon current steering using EO switches, such as can be fabricated on a semi-insulating (Fe doped) InP substrate. Such EO bridge type sample and hold circuitry offers high charging capability, commanding signal isolation, and reduced time jitter.